1. Field of the Invention
The present invention relates to electrically conducting polymer mixtures and methods of preparing them, particularly to electrically conducting polymer mixtures which are mixtures of a matrix polymer and an electrically conductive polymer. More particularly, the present invention relates to electrically conductive polymer mixtures where the matrix polymer is a thermoset and the conducting polymer is protonated polyaniline.
2. Description of the Related Art
Electrically conductive polymers are at present subject to great interest in different parts of the world. These polymers can be used for replacing metal conductors and semi-conductors in a number of applications, such as batteries, sensors, switches, light cells, circuit boards, heating elements, electrostatic discharge elimination (ESD), and electromagnetic interference shielding (EMI). The advantages of conductive polymers over metals include their light weight, mechanical properties, corrosion resistance, and less expensive synthesis and processing methods.
Electrically conducting polymer compositions are of increasing practical interest, for instance, in packaging of electronic instruments and parts, and in solving a wide range of static discharge, electrostatic dissipation and electromagnetic shielding problems.
Electrically conductive plastics can be divided roughly into two categories: filled conductive plastics, in which a conductive filler, such as carbon black or soot, carbon fiber, metal powder, etc., is added to a thermosetting or thermoplastic resin, and intrinsically conductive plastics, which are based on polymers which have been rendered electrically conductive by oxidation, reduction or protonation (doping).
Often, the filled conductive plastics are made by mixing solid conductive particles, such as carbon black, stainless steel fibers, silver or aluminum flakes, or nickel-coated glass fibers, with an insulating plastic material. The plastic material can be either a thermoplastic material, e.g., polystyrene, polyolefin, nylons, polycarbonate, acrylonitrile-butadiene-styrene co-polymers (ABS), and the like or a thermoset, e.g., polyesters or phenolic resins.
The electrical conductivity of filled conductive polymers is dependent on mutual contacts between the conductive filler particles. Usually a well dispersed filler is needed in amounts of approximately 10-50 wt. % to produce composites having a good conductance. However, such conductive composites involve problems: their mechanical and certain of their chemical properties are crucially impaired as the filler content increases and the polymer content decreases; their conductivity is difficult to control, especially within the semiconductor range; and stable and homogenous dispersing of the filler into the matrix plastic is difficult.
Major problems related to these so-called "filled" electrically conductive plastic compositions include difficulties in processing techniques, and the often poor mechanical properties, such as brittleness and reduced elongation at break, of the final products. Also problems with colorability and poor adjustability of conducting properties are characteristic of filled electrically conducting plastic materials.
More recently, there has been increased interest in replacing the above-mentioned carbon black or metal particle-filled compounds with intrinsically electrically conductive polymers, and compositions thereof with common insulating polymers. Intrinsically conductive plastics can be prepared from organic polymers containing long conjugated chains formed by double bonds and heteroatoms. The polymers can be rendered conductive by modifying the .pi.- and .pi.-p-electron systems in their double bonds and heteroatoms by adding to the polymer certain blending or doping agents which will serve as electron receptors or electron donors in the polymer. Thereby electron holes or extra electrons are formed in the polymer chain, enabling electric current to travel along the conjugated chain.
An advantage of the intrinsically conductive plastics is the ease of varying their conductivity as a function of the amount of the doping agent, i.e. the degree of doping, especially within low conductivity ranges. On the other hand, achieving low conductivities with filled conductive plastics is difficult. Examples of currently known intrinsically conductive polymers include polyacetylene, poly-p-phenylene, polypyrrole, polythiophene and its derivatives, and polyaniline and its derivatives.
Among the various conductive polymers, polyanilines in particular have attracted special attention because of their excellent environmental stability and their low production costs.
Polyaniline is well known in the art, and its synthesis and the preparation of its electrically conductive form by, for example, protonating it to the "doped" form by using strong protonic acids, has been disclosed. Typical examples of protonic acids used in the protonation of polyaniline are HCl, H.sub.2 SO.sub.4, sulfonic acids, phosphoric acids, etc. See, for instance, U.S. Pat. Nos. 5,069,820; 5,232,631; and 5,160,457; each of which are hereby incorporated by reference.
Polyaniline, with its derivatives, is in particular a technically and commercially promising intrinsically conductive polymer. An aniline polymer or a derivative thereof is made up of aniline monomers or derivatives thereof, the nitrogen atom of which is bonded to the paracarbon of the benzene ring of the subsequent unit. Unsubstituted polyaniline may appear in a number of forms, including leucoemeraldine, protoemeraldine, emeraldine, nigraline, and toluprotoemeraldine forms. For conductive polymer applications, the emeraldine form is generally used, having the formula ##STR1## wherein X is approximately 0.5.
According to state-of-the-art technology, the doping of polyaniline is usually carried out by using protonic acids, which include HCl, H.sub.2 SO.sub.4, HNO.sub.3, HClO.sub.4, HBF.sub.4, HPF.sub.6, HF, phosphoric acids, sulfonic acids, picrinic acid, n-nitrobenzoic acid, dichloroacetic acid, and polymer acids.
Preferably the prior art doping is carried out using sulfonic acid or its derivatives, such as dodecylbenzenesulfonic acid (DBSA). The protonization is focused on the iminic nitrogen atoms in the aniline units according to the formula presented above, which comprise approximately 50% of the N atoms of polyaniline. Examples of publications in the field include U.S. Pat. Nos. 3,963,498, 4,025,463, and 4,983,322, which are hereby incorporated by reference. The doping of polyaniline with protonic acids is also widely discussed in the literature in the field. U.S. Pat. No. 5,171,478, which is hereby incorporated by reference, discloses a method for increasing the molar mass of polyaniline by heating the polyaniline until its viscosity has increased.
Polyanilines have been blended successfully with thermoplastic polymers. Several patent publications describe electrically conducting thermoplastic polymer compounds and blends which can be solution and melt-processed. See, for instance, U.S. Pat. No. 5,232,631 and EP 582,919, which are hereby incorporated by reference. U.S. Pat. No. 5,232,631 discloses processible polyaniline compositions and blends that exhibit much lower percolation thresholds, sometimes even below 1% w/w, of conductive polyaniline. The patent relates to conductive polymers and particularly to the use of functionalized protonic acids to induce processibility of electrically conductive polyanilines, and to induce solubility of electrically conductive polyanilines in organic liquids or fluid (melt) phases of solid thermoplastic polymers.
According to the prior art, electrically conducting thermosets contain carbon black, metal fibers, flakes, etc. These kinds of filled conducting compositions have, however, the above-mentioned defects due to, for instance, impaired mechanical properties.
Patent publication WO 93/14166, which is incorporated herein by reference, discloses an anticorrosive paint comprising a binder and one or more neutral or electrically conductive conjugated homopolymers or copolymers, such as doped polyaniline. As examples of thermoset resin binders, unsaturated polyester resins and phenolic resins can be used. Binders and conductive polymers have been dispersed in liquid medium, such as water or organic hydrocarbons to form dispersions. In such dispersions, it is not possible to guarantee uniform or steady conductivity. In paints prepared according to this publication, no uniform "cross-linked net-works" are formed by doped polyaniline.